Methods and apparatus for a track and hold amplifier

ABSTRACT

Various embodiments of the present technology may comprise methods and apparatus for a track-and-hold amplifier configured to sample and amplify an analog signal. Methods and apparatus for a track-and-hold amplifier according to various aspects of the present invention may comprise an isolation circuit configured to isolate transient current in a track-and-hold capacitor during a track phase. According to various embodiments, selective activation of the isolation circuit provides a settling time that is independent of the gain of the amplifier.

BACKGROUND OF THE TECHNOLOGY

Amplifiers are utilized in a wide variety of electronic devices and systems to amplify or attenuate a signal. Track-and-hold amplifiers, such as the amplifier illustrated in FIG. 8, are commonly used in data acquisition systems because they are capable of sampling/tracking an analog signal and holding the value during some other operation, such as analog-to-digital conversion.

In various applications, the settling time of the output signal of the amplifier is an important parameter and may affect the overall performance of the electronic device. For example, in high frame rate and/or high resolution imaging applications, a shorter settling time of the output signal is required to provide the desired frame rate and/or resolution. In addition, in applications that utilize multiple amplifiers, a mismatch in settling time between the amplifiers may manifest itself as noise in the signal, which may adversely affect the final signal.

In general, the settling time may be reduced by increasing the supply current and/or decreasing the size of the sampling capacitor. Decreasing the size of the sampling capacitor, however, increases the kTC (sampling) noise, while increasing the supply current increases the overall power consumption of the device.

SUMMARY OF THE INVENTION

Various embodiments of the present technology may comprise methods and apparatus for a track-and-hold amplifier configured to sample and amplify an analog signal. Methods and apparatus for a track-and-hold amplifier according to various aspects of the present invention may comprise an isolation circuit configured to isolate transient current in a track-and-hold capacitor during a track phase. According to various embodiments, selective activation of the isolation circuit provides a settling time that is independent of the gain of the amplifier.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A more complete understanding of the present technology may be derived by referring to the detailed description when considered in connection with the following illustrative figures. In the following figures, like reference numbers refer to similar elements and steps throughout the figures.

FIG. 1 is a block diagram of a system in accordance with an exemplary embodiment of the present technology;

FIG. 2 is a circuit diagram of a single-ended input track-and-hold amplifier in accordance with an exemplary embodiment of the present technology;

FIG. 3 is a timing diagram of the single-ended input track-and-hold amplifier in accordance with an exemplary embodiment of the present technology;

FIG. 4 is an alternative timing diagram of the single-ended input track-and-hold amplifier in accordance with an exemplary embodiment of the present technology;

FIG. 5 is a circuit diagram of a differential input track-and-hold amplifier in accordance with an exemplary embodiment of the present technology;

FIG. 6 is a timing diagram of a differential input track-and-hold amplifier in accordance with an exemplary embodiment of the present technology;

FIG. 7 illustrates output waveforms of a track and hold-amplifier in accordance with an exemplary embodiment of the present technology and a conventional track-and-hold amplifier; and

FIG. 8 is a circuit diagram of a conventional track-and-hold amplifier.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present technology may be described in terms of functional block components and various processing steps. Such functional blocks may be realized by any number of components configured to perform the specified functions and achieve the various results. For example, the present technology may employ various controllers, signal converters, switching devices, current sources, and semiconductor devices, such as transistors, capacitors, and the like, which may carry out a variety of functions. In addition, the present technology may be integrated in any number of electronic systems, such as imaging, automotive, aviation, “smart devices,” portables, and consumer electronics, and the systems described are merely exemplary applications for the technology.

Referring to FIG. 1, an imaging system 100 may be configured to capture image data by converting charge generated by the light into a voltage, which is used to form a digital image. The imaging system 100 may employ various circuits and/or systems to convert light into a digital image. For example, the imaging system 100 may comprise a pixel array 105 comprising a plurality of pixels 110 arranged in rows and columns to form a pixel array 105. The pixel array 105 may contain any number of rows and columns, for example, hundreds or thousands of rows and columns. In an exemplary embodiment, each pixel 110 comprises a photodetector, such as a photodiode, for capturing light and converting light into an electric signal.

In various embodiments, the imaging system 100 may further comprise row circuitry 115, column circuitry 120, and a timing and control unit 125, for selectively activating sequential rows of pixels and transmitting pixel signals out of the pixel array 105 (i.e., read out). The row circuitry 220 may receive row addresses from the timing and control unit 125 and supply corresponding row control signals, such as reset, row select, charge transfer, and readout control signals to the pixels 110 over a row control path to the pixels 110 in the pixel array 105.

The column circuitry 210 may comprise column control circuitry, analog-to-digital conversion circuitry, readout circuitry, processing circuitry, and/or column decoder circuitry, and the like, and may receive pixel signals, such as analog pixel signals generated by the pixels 110. Column paths 145 may couple each column of the pixel array 105 to the column circuitry 120. The column paths 145 may be used for reading out pixel signals from the pixels 110 and/or supplying bias signal (e.g., bias current or bias voltages).

Each pixel signal may be transmitted to a signal conversion circuit, such as an analog-to-digital converter (ADC) (not shown). According to various embodiments, the ADC may be selected according to the particular application. For example, the ADC may be configured as a ramp ADC, a SAR (successive approximation register) ADC, a delta-sigma ADC, or any other suitable ADC architecture. The digital pixel data may then be transmitted to and stored in the image signal processor (ISP) 135 for further processing, such as image reconstruction, white balancing, noise reduction, color correction, and the like.

The imaging system 100 may further comprise a track-and-hold amplifier circuit (T/H amp) 140(A/B) configured to sample and amplify an output signal, such as the pixel signal. The T/H amp 140(A/B) may be interposed on the column paths 145 such that each column of the pixel array 105 has a corresponding T/H amp 140(A/B). The T/H amp 140(A/B) may amplify the pixel signal received from an associated column in the pixel array 105, and may provide the received pixel signal with a desired gain (e.g., a dynamic adjustable gain). In various embodiments, the T/H amp 140(A/B) may apply a relatively high gain to the pixel signals, such as when the corresponding pixel captures darker portions of a scene, or may apply a relatively low gain, such as when the corresponding pixel captures brighter portions of a scene. The column circuitry 210 may provide control signals to the T/H amp 140(A/B) to control the gain of the pixel signals.

Referring to FIGS. 2 and 5, the T/H amp 140(A/B) may be configured to receive an input signal V_(IN), such as the pixel output, and transmit an output signal V_(OUT) to a secondary circuit, such as the column circuitry 120. The T/H amp 140(A/B) may comprise a gain setting element 215, a first current source I₁, a first switch S₁, a track-and-hold capacitor C_(L), a cascode circuit 220, and an isolation circuit 200. The T/H amp 140(A/B) may be configured as a singled-ended input amplifier, for example a single-ended T/H amp 140A as illustrated in FIG. 2, or as a differential amplifier, for example a differential T/H amp 140B as illustrated in FIG. 5. According to various embodiments, the T/H amp 140(A/B) may be coupled to a voltage supply V_(DD).

The cascode circuit 220 may be configured to have a high input impedance, high output impedance, and/or high open-loop gain according to the particular application. In an exemplary embodiment, the cascode circuit 220 is coupled to the first current source I₁ at a first node N₁ and to a reference voltage, such as a ground voltage GND. For example, the cascode circuit 220 may comprise a first transistor M₁ and a second transistor M₂ coupled in series. In an exemplary embodiment, the first and second transistors M₁, M₂ are n-type transistors. The first transistor M₁ may be coupled to the input signal V_(IN) via the gain setting element 215 and to the ground GND, for example, a gate terminal may be coupled to the input voltage and a source terminal may be coupled to the ground GND. The second transistor M₂ may be coupled to the first current source I₁. The second transistor M₂ may be coupled to the first current source I₁ and a bias voltage V_(B), for example, a drain terminal may be coupled to the first current source I₁ and a gate terminal may be coupled to the bias voltage V_(B). The cascode circuit 220 may further comprise a fourth node N₄ having a connection point positioned between the first and second transistors M₁, M₂, which also connects one or more other components and/or circuits to the cascode circuit 220.

The first current source I₁ may provide a bias current to one or more components of the T/H amp 140(A/B). For example, in an exemplary embodiment, the first current source I₁ is coupled to the second transistor M₂ of the cascode circuit 220. The first current source I_(I) may comprise any circuit and/or system suitable for providing a bias current. The value of the current produced by the first current source I_(I) may be selected according to the specific application, power consumption limitations, and the like.

The gain setting element 215 may be configured to apply a predetermined gain A to the input signal V_(IN). The gain setting element 215 may comprise any suitable circuit and/or system to control the gain of the T/H amp 140(A/B). In various embodiments, the gain setting element 215 may comprise one or more impedance elements, such as capacitors, variable resistors, and/or a combination thereof. For example, in an exemplary embodiment, the gain setting element 215 may comprise a first impedance element, such as a first capacitor C_(I) and a second impedance element, such as a second capacitor C_(F) coupled in series, where the gain A of the T/H amp 140(A/B) is equal to the value of the first capacitor C_(I) divided by the value of the second capacitor C_(F) (i.e., A=C_(I)/C_(F)). The gain setting circuit 215 may further comprise a second node N₂ positioned between the first and second capacitors C_(I), C_(F). The second node N₂ may be used as a connection point for other circuits and/or components within the T/H amp 140(A/B). The gain setting element 215 may introduce a first time constant T_(C1) described by: T_(C1)=(1+A)*(C_(I)C_(F)/(C_(I)+C_(F)))/g_(m), where g_(m) is the transconductance of the T/H amp 140(A/B).

The track-and-hold capacitor C_(L) may be configured to track the input signal V_(IN) and hold a value for a period of time. The track-and-hold capacitor C_(L) may comprise a first plate (i.e., a first capacitor terminal) and a second plate (i.e., a second capacitor terminal). In an exemplary embodiment, the first plate of the track-and-hold capacitor C_(L) may be selectively coupled to the first current source I₁ via the first switch S₁.

Referring to FIG. 8, a track-and-hold capacitor of a conventional track and hold amplifier may introduce a second time constant T_(C2) described by the following: T_(C2)=(1+A)*C_(L)/g_(m). In the conventional T/H amplifier, the second time constant T_(C2) further increases the settling time of the output signal V_(OUT).

The isolation circuit 200 may be configured to isolate any transient current in the track-and-hold capacitor C_(L). The isolation circuit 200 may comprise any number of components, such as transistors, resistors, switching devices, clamping devices, current shunts, and the like, suitable for isolating a desired current. For example, in an exemplary embodiment, the isolation circuit 200 comprises a second current source I₂, a third transistor M₃, a second switch S₂, and a clamp device 210 that operate in conjunction with each other to isolate current from the track-and-hold capacitor C_(L). In various embodiments, the isolation circuit 200 may be coupled to the voltage supply V_(DD), the track-and-hold capacitor C_(L), and the cascode circuit 220.

According to an exemplary embodiment, the third transistor M₃ is a p-type transistor. The third transistor M₃ may be coupled to the second plate of the track-and-hold capacitor C_(L) and to the second current source I₂ at a third node N₃. For example, a source terminal of the third transistor M₃ may be coupled to the second plate of the track-and-hold capacitor C_(L) as well as to the second current source I₂. The third transistor M₃ may be further coupled to the second switch S₂, the clamping device 210, and the cascode circuit 220. For example, a drain terminal of the third transistor M₃ may be coupled to the second switch S₂, which selectively couples the third transistor to the ground GND, to the clamping device 210, and to the cascode circuit 220 via the fourth node N₄.

A gate terminal of the third transistor M₃ may be coupled to a dynamic voltage V_(D), which may change values during the operation of the T/H amp 140(A/B). The values of the dynamic voltage V_(D) may be selected according to the particular application, such as the particular type of signal converter architecture and other relevant factors. For example, the dynamic voltage V_(D) may supply a DC voltage during a tracking period and may supply a ramp voltage during a holding and/or conversion period.

The third transistor M₃ may introduce a third time constant T_(C3) at the output given by: T_(C3)=C_(L)/g_(m3), where g_(m3) is the transconductance of the third transistor M₃. The gain A of the T/H amp 140(A/B) has no effect on the third time constant T_(C3), so even at high gains, there is no degradation of the settling time.

The second current source I₂ may provide a bias current to one or more components of the T/H amp 140(A/B). For example, in an exemplary embodiment, the second current source I₂ is coupled to the third transistor M₃, such as a source terminal of the third transistor M₃. The second current source I₂ may comprise any circuit and/or system suitable for providing a bias current. The value of the current produced by the second current source I₂ may be selected according to the specific application, power consumption limitations, and the like. According to an exemplary embodiment, the current value of the second current source I₂ is equal to the current value of the first current source I₁ (i.e., I₁=I₂).

The clamp device 210 provides a current path from the drain terminal of the third transistor M₃ to the ground GND when the first transistor M₁ is off. The clamp device 210 may comprise any suitable component having a clamping voltage greater than the steady state drain-to-source voltage V_(DS) of the first transistor M₁. For example, in an exemplary embodiment, the clamp device 210 may comprise a Zener diode. In alternative embodiments, however, the clamp device 210 may comprise a diode, a diode-connected transistor, and the like. In an exemplary embodiment, the clamp device 210 is coupled between the fourth node N₄ of the cascode circuit 220 and the ground GND. The clamp device 210 is positioned such that a cathode terminal of the clamp device 210 is coupled to the fourth node N₄ and the anode terminal of the clamp device 210 is coupled to the ground GND.

Referring to FIG. 5, the differential T/H amp 140B may be configured to receive the input signal V_(IN) as well as a reference voltage V_(REF). The reference voltage V_(REF) may be any voltage level and may be selected according to the particular application. The differential T/H amp 140B may further comprise a fourth transistor M₄, a fifth transistor M₅, a sixth transistor M₆, and a third current source I₃.

The third current source I₃ may provide a bias current to one or more components of the differential T/H amp 140B. For example, in an exemplary embodiment, the third current source I₃ is coupled to the fourth and fifth transistors M₄, M₅. The third current source I₃ may comprise any circuit and/or system suitable for providing a bias current. The value of the current produced by the third current source I₃ may be selected according to the specific application, power consumption limitations, and the like. According to an exemplary embodiment, the third current source I₃ may be equal to the first current plus the second current, multiplied by two (i.e., I₃=2(I₁+I₂)).

The fourth, fifth, and sixth transistors M₄, M₅, M₆ may be configured to provide a current path in response to the reference voltage V_(REF). In an exemplary embodiment, the fourth and fifth transistors M₄, M₅ are p-type transistors and the sixth transistor M₆ is an n-type transistor. In an exemplary embodiment, the fourth and fifth transistors M₄, M₅ may be coupled in parallel with each other and coupled to the third current source I₃. For example, source terminals of the fourth and fifth transistors M₄, M₅ may be coupled to the third current source I₃, a drain terminal of the fifth transistor M₅ may be coupled to the ground GND, a gate terminal of the fifth transistor M₅ may be coupled to the gain setting element 215, and a gate terminal of the fourth transistor M₄ may be coupled to the reference voltage V_(REF). The sixth transistor M₆ may be coupled in series with the fourth transistor M₄ and the ground GND, for example the drain terminals of the fourth and sixth transistor M₄, M₆ may be coupled together, and the gate terminal of the sixth transistor M6 may be coupled to the gate terminal of the first transistor M₁ and its drain terminal.

According to various embodiments, the first and second switches S₁, S₂ may be responsive to a control signal transmitted from a control unit, such as the timing and control unit 125 and/or any other circuit suitable for transmitting control signals. For example, the control signal may open or close the switches at a predetermined time and/or interval. The operation of the first and second switches S₁, S₂ may be predetermined according to the particular application and/or environment. The first and second switches S₁, S₂ may comprise any suitable circuit and/or device capable of coupling/decoupling one or more components of the T/H amp 140(A/B).

According to various embodiments, the T/H amp 140(A/B) isolates transient current in the track-and-hold capacitor C_(L) during a track phase, which decreases the settling time of the output signal V_(OUT). According to various embodiments, the T/H amp 140(A/B) utilizes the isolation circuit 200 to redirect current from the track-and-hold capacitor C_(L) to the first transistor M₁ to ensure that the current through the first transistor M₁ is constant. By redirecting the current, the settling time is based on the time constant of a series value of the first and second capacitors C_(I), C_(F) (i.e., (C_(I)C_(F))/(C_(I)+C_(F)) (rather than the track-and-hold capacitor C_(L), as is the case in a conventional T/H amplifier), which is much smaller than the time constant of the track-and-hold capacitor C_(L) since the series value of the first and second capacitors C_(I), C_(F) is much smaller than the capacitance value of the track-and-hold capacitor C_(L). For example, and referring to FIGS. 2, 7, and 8, given identical total supply current and capacitor values for the first, second, and track-and-hold capacitors C_(I), C_(F), C_(L), the settling time of the output signal V_(OUT) for the exemplary T/H amp 140(A/B) is approximately 250 ns (where I₁+I₂=8 μA, C_(L)=1 pF, C_(I)=0.8 pF, C_(F)=0.1 pF, A=8), while the settling time for the conventional T/H amplifier is approximately 700 ns (where I=8 μA, C_(L)=1 pF, C_(I)=0.8 pF, C_(F)=0.1 pF, A=8).

Referring to FIGS. 2 and 4, in an exemplary operation, the T/H amp 140(A/B) may operate according to a track phase and a hold phase. According to exemplary embodiment, during the track phase, the first switch S₁ is closed (ON) and the second switch S₂ is open (OFF). Additionally, the first and second current sources are also ON (i.e., I₁≠0, I₂≠0), and the dynamic voltage V_(D) is held at a predetermined constant value, such as 1V. This causes the current through the track-and-hold capacitor C_(L), which is connected to the source terminal of the third transistor M₃, to flow back into the first transistor M₁ via the fourth node N₄. As such, the current through the first transistor M₁ is independent of the current through the track-and-hold capacitor C_(L), and therefore the second time constant T_(C2) of the track-and-hold capacitor C_(L) has no effect on the settling time of the output signal V_(OUT). Rather, the settling time of the output signal V_(OUT) depends on the first time constant T_(C1), which is much less than the second time constant T_(C2). During the track phase, the output signal V_(OUT) may be inverted with respect to the input signal V_(IN).

During the hold phase, the first switch S₁ is opened (OFF) and the track-and-hold capacitor C_(L) holds the value of output signal V_(OUT). The second switch S₂ is then closed (ON). According to various embodiments, the first current source I₁ may also be turned off to conserve power after the first switch S₁ is opened and either before or after the second switch S₂ is closed. After the second switch S₂ is closed (ON), the dynamic voltage V_(D) may provide a ramp signal or other appropriate signal and the output signal V_(OUT) follows the dynamic voltage V_(D). For example, and referring to FIG. 3, the dynamic voltage V_(D) may maintain a constant value, such as 2V, during both the track phase and the hold phase. The behavior of the dynamic voltage V_(D) during the hold phase may be selected according to the particular application.

Referring to FIGS. 5 and 6, the differential T/H amp 140B operates in the same manner as described above. Additionally, the third current source I₃ has the same timing waveform as the first current source I₁, as described above. When the third current source I₃ is turned on, the fourth, fifth, sixth transistors M₄, M₅, M₆ conduct current.

According to various operations, the first and third transistors M₁, M₃ may go into a cut-off mode, wherein the transistors act like open circuits. In the case where the first transistor M₁ goes into cut-off mode, the clamp device 210 provides a current path around the first transistor M₁ to the ground GND. Further, in such a case, if the first current I₁ is equal to the second current I₂, then the T/H amp(A/B) has a symmetric slew rate SR described by: SR=C_(L)*ΔV_(OUT)/I₁, where ΔV_(OUT) is a step difference in the output signal V_(OUT). Even though the settling time of the output signal V_(OUT) may increase due to the finite slew rate, the settling time of the output signal V_(OUT) of the T/H amp 140(A/B) is still much less than a conventional T/H amplifier and is almost independent of the gain A.

In the foregoing description, the technology has been described with reference to specific exemplary embodiments. The particular implementations shown and described are illustrative of the technology and its best mode and are not intended to otherwise limit the scope of the present technology in any way. Indeed, for the sake of brevity, conventional manufacturing, connection, preparation, and other functional aspects of the method and system may not be described in detail. Furthermore, the connecting lines shown in the various figures are intended to represent exemplary functional relationships and/or steps between the various elements. Many alternative or additional functional relationships or physical connections may be present in a practical system.

The technology has been described with reference to specific exemplary embodiments. Various modifications and changes, however, may be made without departing from the scope of the present technology. The description and figures are to be regarded in an illustrative manner, rather than a restrictive one and all such modifications are intended to be included within the scope of the present technology. Accordingly, the scope of the technology should be determined by the generic embodiments described and their legal equivalents rather than by merely the specific examples described above. For example, the steps recited in any method or process embodiment may be executed in any order, unless otherwise expressly specified, and are not limited to the explicit order presented in the specific examples. Additionally, the components and/or elements recited in any apparatus embodiment may be assembled or otherwise operationally configured in a variety of permutations to produce substantially the same result as the present technology and are accordingly not limited to the specific configuration recited in the specific examples.

Benefits, other advantages and solutions to problems have been described above with regard to particular embodiments. Any benefit, advantage, solution to problems or any element that may cause any particular benefit, advantage or solution to occur or to become more pronounced, however, is not to be construed as a critical, required or essential feature or component.

The terms “comprises”, “comprising”, or any variation thereof, are intended to reference a non-exclusive inclusion, such that a process, method, article, composition or apparatus that comprises a list of elements does not include only those elements recited, but may also include other elements not expressly listed or inherent to such process, method, article, composition or apparatus. Other combinations and/or modifications of the above-described structures, arrangements, applications, proportions, elements, materials or components used in the practice of the present technology, in addition to those not specifically recited, may be varied or otherwise particularly adapted to specific environments, manufacturing specifications, design parameters or other operating requirements without departing from the general principles of the same.

The present technology has been described above with reference to an exemplary embodiment. However, changes and modifications may be made to the exemplary embodiment without departing from the scope of the present technology. These and other changes or modifications are intended to be included within the scope of the present technology, as expressed in the following claims. 

1. A track-and-hold amplifier circuit configured to be coupled to a supply voltage and a first reference voltage and receive an input signal, comprising: a gain setting element coupled to the input signal; a cascode circuit coupled to the gain setting element; a track-and-hold capacitor comprising: a first capacitor terminal; and a second capacitor terminal; and an isolation circuit, coupled to the track-and-hold capacitor and the cascode circuit, and configured to selectively isolate transient current in the track-and-hold capacitor and redirect the transient current through at least a portion of the cascode circuit.
 2. The track-and-hold amplifier according to claim 1, further comprising a first current source, wherein the first current source is: coupled to the cascode circuit and the gain setting element at a first node; and selectively coupled to the track-and-hold capacitor.
 3. The track-and-hold amplifier according to claim 1, wherein the isolation circuit comprises: a third transistor comprising: a first terminal; and a second terminal; wherein: the first terminal is coupled to the second capacitor terminal at a third node; and the second terminal is coupled to the cascode circuit; a second current source coupled to the third node; and a switch coupled between the second terminal and the first reference voltage.
 4. The track-and-hold amplifier according to claim 3, wherein: the third transistor further comprises a third terminal coupled to a dynamic voltage; the second terminal is coupled to the cascode circuit via a fourth node.
 5. The track-and-hold amplifier according to claim 3, wherein the switch selectively couples the second terminal: to the first reference voltage during a hold phase; and to the cascode circuit via a fourth node during a track phase.
 6. The track-and-hold amplifier according to claim 1, wherein: the cascode circuit comprises: a first transistor; and a second transistor; wherein a fourth node is positioned between the first and second transistors; and the isolation circuit comprises a clamp device coupled to the fourth node and the first reference voltage.
 7. The track-and-hold amplifier according to claim 6, wherein the clamp device has a clamping voltage that is greater than a drain-to-source voltage of the first transistor.
 8. The track-and-hold amplifier according to claim 1, wherein the gain setting element comprises: a first impedance element coupled in series with a second impedance element; and a second node positioned between the first and second impedance elements; wherein: each impedance element may comprise at least one of a resistor and a capacitor; and a series capacitance value of the first and second impedance elements is less than a capacitance value of the track-and-hold capacitor.
 9. The track-and-hold amplifier according to claim 1, wherein the cascode circuit is coupled to: the gain setting element via a second node; and a bias voltage.
 10. The track-and-hold amplifier according to claim 1, wherein the isolation circuit comprises a clamp device configured to provide a current path from the isolation circuit to the first reference voltage when at least a portion of the cascode circuit is off.
 11. The track-and-hold amplifier according to claim 1, further comprising: a third current source; a fourth transistor coupled to the third current source and a second reference voltage; and a fifth transistor coupled to the third current source, the input signal, and the first reference voltage.
 12. A method for improving an output signal settling time for a track-and-hold amplifier, comprising: during a tracking phase: supplying a first current to a cascode circuit comprising a first transistor and a second transistor; tracking an input signal with a track-and-hold capacitor comprising a first terminal and a second terminal; supplying a second current to a third transistor coupled to the second terminal of the track-and-hold capacitor; and directing a transient current through the track-and-hold capacitor to a reference voltage through one of: the first transistor and a clamp device.
 13. The method according to claim 10, further comprising: during a holding phase: selectively coupling the third transistor to the reference voltage; stopping the first current; and supplying a dynamic voltage to the third transistor.
 14. An imaging system, comprising: a plurality of pixel columns; a plurality of track-and-hold amplifiers, wherein: each pixel column is coupled to one track-and-hold amplifier; and each track-and-hold amplifier comprises: a gain setting element coupled to the input signal; a cascode circuit, coupled to the gain setting element at a second node and a bias voltage, comprising:  a first transistor; and  a second transistor; a track-and-hold capacitor, comprising: a first terminal; and a second terminal; a first current source, coupled to the cascode circuit and the gain setting element at a first node, and selectively coupled to the track-and-hold capacitor; a third transistor, comprising: a first terminal; a second terminal; and a third terminal; wherein:  the first terminal is coupled to the second terminal at a third node;  the second terminal is coupled to the cascode circuit at a fourth node; and  the third terminal is coupled to a dynamic voltage; a switch coupled between the second terminal and a reference voltage; and a second current source coupled to the third node.
 15. The imaging system according to claim 14, wherein: wherein the switch selectively couples the second terminal: to the reference voltage during a hold phase; and to the cascode circuit via a fourth node during a track phase.
 16. The imaging system according to claim 14, wherein the gain setting element comprises: a first impedance element coupled in series with a second impedance element; and a second node positioned between the first and second impedance elements; wherein: each impedance element may comprise at least one of a resistor and a capacitor; and a series capacitance value of the first and second impedance elements is less than a capacitance value of the track-and-hold capacitor.
 17. The imaging system according to claim 14, further comprising a ramp ADC configured to receive an output signal from each track-and-hold amplifier, and wherein the dynamic voltage is a ramp voltage.
 18. The imaging system according to claim 14, further comprising a clamp device coupled between the fourth node and the reference voltage, wherein the clamp device has a clamping voltage that is greater than a drain-to-source voltage of the first transistor.
 19. The imaging system according to claim 14, further comprising a clamp device configured to provide a current path from the third transistor to the reference voltage when the first transistor is off.
 20. The imaging system according to claim 14, further comprising: a third current source; a fourth transistor coupled to the third current source and a reference voltage; and a fifth transistor coupled to the third current source. 